1. Field of the Invention
The present invention relates to a process for producing inorganic molds as ceramic products, and specifically it relates to a process for producing inorganic molds with satisfactory dispersion stability of the inorganic particles in the kneaded slurry material, and good plasticity, fillability and shape retention.
2. Description of the Related Art
Conventionally known inorganic molds including metal founding sand-cast compositions, amorphous refractories, refractory molds such as fire bricks, etc. and ceramics, are produced by working the pulverized inorganic material together with a water-soluble polymer into a kneaded slurry, forming it into a mold by casting, compression molding, extrusion molding or injection molding, and drying or firing and cooling the mold.
The water-soluble polymer used here must be one which both imparts suitable water retentivity to the inorganic substance and increases the plasticity of the kneaded slurry even when used in small amounts, and which provides satisfactory dispersion stability of the inorganic material particles in the kneaded mixture while improving the wet strength, dry strength and firing strength of the mold. Water-soluble cellulose esters have been the main water-soluble polymers used in the past.
However, although water-soluble cellulose ethers with large average molecular weights can increase the plasticity of kneaded mixtures even in small amounts, their effect is minimal with respect to imparting water retentivity. Conversely, water-soluble cellulose ethers with low average molecular weights must be used in large amounts, which is undesirable due to their adverse effect on the strength and water resistance of the resulting product, and also higher cost.
Further, the effect of water-soluble cellulose ethers of imparting dispersion stability to the inorganic material particles is based mainly on thickening of the water, and is thus minimal. Consequently, while water retentivity and increased plasticity of kneaded mixtures have been achieved by the effect of water-soluble cellulose polymers alone, it has been difficult to ensure dispersion stability of the inorganic particles.
In recent years, microorganically produced polysaccharides, such as pullulan, have been used as water-soluble polymers aimed at overcoming the problems mentioned above, but because these are exceedingly costly, they have not come into general use.
As stated above, though water-soluble polymers as components of inorganic molds must impart pre-drying and pre-firing dispersion stability in the kneaded slurry, plasticity, fillability, shape retention and strength, not all of these properties are provided under the present circumstances.
It is an object of the present invention to develop a process for producing inorganic molds of which the inorganic material particles are stable in dispersion, and which thus have satisfactory plasticity, fillability and shape retention, and which may also be provided cheaply and consistently.
As a result of diligent research in light of these circumstances, the present inventors have arrived at the discovery that inorganic molds with satisfactory dispersion stability, plasticity, fillability and shape retention may be obtained by using water-soluble hemicellulose, particularly pulse-derived water-soluble hemicellulose, as the water-soluble polymer. The present invention has been completed on the basis of this discovery.
In other words, the present invention provides a process for producing inorganic molds by molding a kneaded mixture containing a particulate inorganic material and water, which process comprises adding water-soluble hemicellulose to the kneaded mixture.
The water-soluble hemicellulose of the invention is preferably derived from pulse, and especially from soybean, specifically the cotyledon thereof.
Water-soluble hemicellulose of any molecular weight may be used, but the average molecular weight is preferably from a few tens of thousand to a few million, and specifically from 50,000 to one million. The average molecular weight of the water-soluble hemicellulose is determined by the limiting viscosity method whereby the viscosity is measured in a 0.1 M NaNO3 solution, with standard pullulan (available from Showa Denko, KK.) as the standard substance. Here, the uronic acid was measured by the Blumenkrantz method, and the neutral saccharides were measured by GLC after alditol acetating.
Water-soluble hemicellulose may be extracted by water from raw materials containing hemicellulose, or in some cases by heating elution under acidic or alkali conditions, or by elution through decomposition with enzymes. The following is an example of a method for producing water-soluble hemicellulose.
The raw material may be a plant, for example, the shell of an oily seed such as soybean, palm, coconut, corn, cottonseed, etc., usually with the oil and protein removed, or the lees from grains such as rice or wheat, usually with the starch, etc. removed. If soybean is to be used as the raw material, it may be obtained as the bean-curd lees produced during manufacture of tofu, soybean milk or separated soy protein.
The raw material is thermally decomposed under acid or alkali conditions, preferably at a pH near the isoelectric point of the protein, preferably between 80xc2x0 C. and 130xc2x0 C. and more preferably between 100xc2x0 C. and 130xc2x0 C., and after separation of the water-soluble fraction, it is either dried immediately or dried after being subjected to, for example, active carbon treatment, resin adsorption treatment or ethanol precipitation treatment to remove the hydrophobic substances or low molecular substances, to obtain the water-soluble hemicellulose.
According to the present invention, the water-soluble hemicellulose may be used as a water-soluble polymer alone, but it may also be used in combination with known water-soluble polymers to compensate for the deficiencies of those known water-soluble polymers.
Known synthetic water-soluble polymers which may be mentioned include water-soluble acryl resins, water-soluble styrene-acryl resins, water-soluble styrene-maleic acid resins, etc. and their salt compositions. Water-dispersible emulsions of acryl resins, alkyd resins, vinyl resins, polyester resins, styrene resins, maleic acid resins and urethane resins are examples of other known effective synthetic polymers.
As known natural water-soluble polymers there may be mentioned gum arabic, gum tragacanth, carrageenan, xanthan gum, gelatin, casein sodium, guar gum, tare gum, laver, agar, furcellaran, tamarind seed polysaccharide, karaya gum, Hibiscus manihot, pectin, sodium alginate, pullulan, jellan gum, locust bean gum, whey and other albumins, and various different starches. Semi-natural water-soluble polymers include carboxymethyl cellulose (CMC), methyl cellulose (MC), ethyl cellulose (EC), hydroxymethyl cellulose (HMC), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxyethylmethyl cellulose (HEMC), hydroxyethylethyl cellulose (HEEC), hydroxypropylmethyl cellulose (HPMC), hydroxypropylethyl cellulose (HPEC), hydroxyethylhydroxypropyl cellulose (HEHPC), sulfoethyl cellulose, dihydroxypropyl cellulose (DHPC), alginic acid propylene glycol ester, and processed starches including soluble starches.
The effect of the water-soluble hemicellulose of the invention is sometimes further improved by combining it with one or more of the water-soluble polymers mentioned above, and it may compensate for deficiencies of the various water-soluble polymers.
The inorganic substance composing the inorganic mold of the invention may be a one- to four-component compound made with SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O, TiO2, P2O5, B2O3, SO3, etc., and is usually a solid solution, glassy substance or combination thereof, with the inorganic raw materials and amount of water-soluble hemicellulose selected depending on the desired product.
For example, when the inorganic mold used according to the invention is a refractory mold or amorphous refractory, the refractory raw material may be any one commonly used in the fire brick industry, and in the case of silica bricks, red white silica, blue white silica, ganister, white silica, etc. may be used. In the case of zircon and zirconia bricks, a raw material obtained from zircon and baddaleyite minerals may be used. In such cases, the amount of water-soluble hemicellulose used may suitably be in the range of 0.01-1 wt %, and preferably 0.05-0.5 wt %, with respect to the inorganic material.
When the inorganic mold is a ceramic, the raw material may be any one which is commonly used in the ceramic industry, including alumina, silica, zirconia, talc, mica, permiculite, mullite, shirasu, pearlite, feldspar, or a kaolin-based or montmorillonite-based clay. When one of these is used, the amount of water-soluble hemicellulose may suitably be in the range of 1-50 wt %, and preferably 5-30 wt %, with respect to the inorganic material.
When the inorganic mold is a metal founding sand-cast composition, the raw material may be any one which is commonly used in the foundry industry, and a clay such as bentonite or refractory clay may be added to the silica sand or other founding sand. When one of these is used, the amount of water-soluble hemicellulose may suitably be in the range of 0.1-10 wt %, and preferably 0.5-5 wt %, with respect to the inorganic material.
Thus, when water-soluble hemicellulose according to the invention is used as the water-soluble polymer for the raw material of an inorganic mold, a more stable and thoroughly dispersed state may be achieved than by using water-soluble cellulose ethers or pullulan.
Embodiments of the present invention will now be explained by way of the following examples which, however, are merely illustrations and are not intended to restrict the scope of the invention. Throughout the examples, the values for xe2x80x9cpartsxe2x80x9d and xe2x80x9c%xe2x80x9d all weight-based.
Preparation of soybean hemicellulose
To raw bean-curd lees obtained during the production of separated soybean protein there was added twice the amount of water, and the pH was adjusted to 4.5 with hydrochloric acid prior to hydrolysis at 120xc2x0 C. for 1.5 hours. The cooled product was centrifuged (10,000 Gxc3x9730 min) and the supernatant and precipitate were separated. The separated precipitate was washed with an equal weight of water and centrifuged, and the supernatant was combined with the previous supernatant, treated with an active carbon column and then dried to obtain water-soluble hemicellulose (A).
This water-soluble hemicellulose was dissolved in 0.5% saline, reprecipitation was repeated 3 times to an ethanol concentration of 50%, and an ion-exchange resin (Amberlite IR-120B, product of Organo, KK.) was used for desalting to obtain water-soluble hemicellulose (B).
The same process was used without active carbon treatment to obtain water-soluble hemicellulose (C).
The results are summarized below.
The saccharide compositions of the water-soluble hemicelluloses (A), (B) and (C) were then analyzed according to the following method. The uronic acid was measured by the Blumenkrantz method, and the neutral saccharides were measured by GLC according to the alditol acetate method.
The results were as follows.
Formation of refractory molds